This invention concerns methods and apparatus for grinding of workpieces to form components which include both concentric and eccentric cylindrical regions. Such a component is an internal combustion engine crankshaft which includes both concentric journal bearing regions and eccentric crankpins. The invention is applicable to any such workpiece which is to be ground to size and finish, and such workpieces will be referred to herein as composite workpieces.
Historically workpieces of this nature have been ground on two grinding machines. A first grinding machine has been used to grind the concentric cylindrical sections and a second grinding machine has been used to grind the eccentric cylindrical sections.
In the first grinding machine the workpiece is rotated about its primary axis and each of the concentric sections are ground by moving a grinding wheel into a grinding position and removing material from the workpiece to produce a concentric cylindrical region thereon, in a conventional manner.
In the second grinding machine, two approaches have been adopted. In one approach the workpiece has been rotated about the axis of each eccentric region in turn, and conventional cylindrical grinding is employed to remove material so as to produce a cylindrical bearing surface at each eccentric region, just as if the element were a concentric cylindrical region of an overall workpiece. In the second approach, the workpiece has been rotated about its primary axis so that the eccentric regions precess eccentrically around the primary axis of rotation, as for example do the crankpins of a crankshaft when the latter is mounted in and is rotated in an engine. In this second approach the grinding wheel has been aligned with an eccentric region and then been advanced and retracted under computer control in synchronism with the precession of the eccentric region so that the grinding wheel remains in contact therewith at all times. The region is thereby ground at all positions of its precession. As each eccentric region is ground to size, the grinding wheel is withdrawn and the workpiece or grinding wheel is repositioned axially so that another eccentric region is aligned with the grinding wheel, ready for a similar grinding operation at the new location.
A process requiring two grinding machines involves considerable down-time as components are moved from one machine to another.
During any grinding operation, but particularly noticeable when grinding regions of an elongate workpiece (such as a crankshaft) which is supported for grinding at its ends, unwanted eccentricity may be introduced into the workpiece due to forces acting on the workpiece during the grinding process. This eccentricity which is termed runout tends to be more noticeable in concentrically ground cylindrical regions of the workpiece. It often appears as a consequence of stress relieving the workpiece, after grinding, and any eccentricity introduced into concentrically ground journal bearing regions a crankshaft will introduce undesirable wear in the component when mounted and rotated in use.
It is one object of the invention to reduce the machining time required to machine composite workpieces containing both concentric and eccentric regions, such as crankshafts which contain concentric Journal bearing regions and eccentric crankpins.
It is a subsidiary object of the invention to reduce unwanted eccentricity of concentrically ground cylindrical regions of a composite workpiece such as concentric cylindrical bearing regions.
It is a further object to provide a method and apparatus which will not only reduce machining time of composite workpieces but also allows eccentricity correction to be applied to both concentrically and eccentrically ground cylindrical workpiece regions such as the joined bearing regions and the workpiece of an engine crankshaft.
(a) Composite Workpiece Grinding in Accordance With the Invention
A method of grinding a composite workpiece to form a component having concentric and eccentric cylindrical regions, comprises, mounting the workpiece in a computer controlled grinding machine having a grinding wheel the advance and withdrawal of which is controllable by a program entered ito the machine controlling computer, loading a wheelfeed controlling program into the computer, positioning the grinding wheel relative to the workpiece, or the workpiece relative to the grinding wheel, so that the wheel is aligned with a first selected region of the workpiece, advancing the grinding wheel towards the selected region so as to grind the surface thereof, controlling the wheel feed so as to maintain grinding contact between the grinding wheel and the workpiece so as either to grind a concentric cylindrical region thereon or to grind an eccentric cylindrical region thereon as required, information about each selected region of the workpiece being stored in the computer memory which is addressed as appropriate so that the wheel feed can be controlled appropriately depending on whether a selected workpiece region is to be ground as a concentric or an eccentric cylindrical region.
The invention allows a workpiece having both concentric and eccentric cylindrical regions thereon to be ground on a single grinding machine without demounting the workpiece from the machine. This enables a workpiece to be left in place between centres whilst different regions therealong are ground to form cylindrical surfaces some of which are concentric with the primary axis of rotation of the workpiece, and others of which are eccentric relative thereto.
The invention is of particular application to the grinding of crankshafts which have journal bearing regions which have to be ground as cylindrical surfaces concentric with the primary axis of rotation of the crankshaft and crankpins which have to be ground as cylindrical surfaces eccentric to the primary axis of rotation.
(b) Correction For Process Induced Eccentricity
During the grinding of any workpiece, stresses and strains introduced by the grinding process particularly in unsupported regions of the workpiece, can cause a cylindrical region which is being ground to become eccentric relative to the primary workpiece axis. Whilst these eccentricities may be of minor consequence in the case of a significantly eccentric region such as a crankpin of a crankshaft, minor eccentricities in what should be concentric cylindrical surface such as a journal bearing surface of a crankshaft, will result in friction and wear in an engine in which such a crankshaft is mounted.
The invention allows for this problem to be overcome in that the control system can be programmed so as to not only distinguish between concentric and eccentric regions of a workpiece to enable the wheel feed to be controlled accordingly (so as to grind a cylindrical surface concentric with the primary axis of the workpiece or a cylindrical surface which is eccentric thereto), (ie whose axis is spaced from but parallel to the said primary axis) but is also capable of being controlled so as to introduce small advance and retract movements in the wheel feed whilst the grinding wheel is engaged in grinding a concentric cylindrical workpiece region, so as to introduce into the ground region an eccentricity in the cylindrical surface which is equal and opposite to any eccentricity which the grinding process per se introduces into the ground region, so that the two cancel out.
The invention also provides for the control system as aforesaid to be programmed to adjust the wheel feed program instructions or control signals when grinding eccentric cylindrical surfaces, so as to introduce an additional eccentricity which is equal and opposite to any stress induced eccentricity introduced into the eccentric cylindrical surface by the grinding process.
This technique of eccentricity compensation therefore may be extended to include not only the grinding of concentric cylindrical workpiece surfaces but also the grinding of eccentric cylindrical workpiece surfaces.
(c) Setting Up
A method of setting up such a grinding machine for grinding a composite workpiece with compensation for process induced eccentricity to concentric cylindrical regions comprises the steps of cylindrically grinding at least one concentric workpiece region to final form, measuring any unwanted eccentricity introduced by the grinding process together with the angular position(s) thereof in the said at least one workpiece region, and adjusting the wheel feed program, instructions or control signals stored in the computer controlling the grinding machine so that during subsequent workpiece grinding, the wheel feed is controlled so as to grind an equal and opposite eccentric form to that measured, thereby to cancel out process induced eccentricity.
Preferably the measuring of the workpiece region occurs after it has been stress relieved, which may be achieved merely by disengaging the grinding wheel, but may involve removing the workpiece from the grinding machine for gauging, then the careful replacement of the workpiece back in position.
Any unwanted eccentricity introduced into a workpiece by a grinding process will tend to vary from one position to another along the length of the workpiece, and will tend to increase with distance from a supported workpiece end. In consequence for example cylindrical journal bearing regions and crankpins near the centre of a crankshaft which is supported at opposite ends may require a greater degree of eccentricity compensation than is the case for those near to the two supported ends.
Where a plurality of such regions are to be ground at spaced apart positions axially along a workpiece which is supported at opposite ends, the setting up process may be performed for a concentric cylindrical region approximately midway along the said axial length of the workpiece, and proportionately reduced corrections are applied to the wheelfeed program instructions or control signals for similar cylindrical regions which are to be ground and which are displaced from the mid position towards the supported ends of the workpiece.
In an alternative method, it is possible to more fully take into account the variation in unwanted eccentricity introduced by grinding process forces along the axial length an elongate workpiece supported at opposite ends, such that regions towards the centre of the workpiece, tend to require a greater degree of eccentricity compensation during grinding than do those adjacent the supported ends thereof. This alternative method involves the steps of concentrically grinding all of the concentric surfaces of a workpiece without introducing any compensation for unwanted process induced eccentricity thereafter making measurements on the workpiece to determine the extent and angular position of an unwanted eccentricity for each of the ground regions, and separately adjusting the wheelfeed program instructions or control signals used to control the wheel feed during subsequent grinding of each said region of the same of a similar workpiece, so as to compensate individually for each said region for any process-induced eccentricity.
After such a setting up process has been performed and the original workpiece has been reground using the corrected wheelfeed program instructions or control signals a further check on the reground workpiece may be made and second order corrections may be made to the instructions or control signals, before regrinding the original workpiece or grinding further similar workpieces.
Measurements may be made subsequent to each further workpiece grinding to determine whether the corrections made from earlier measurements sufficiently compensate for any unwanted process induced eccentricity, and further corrections may be made to the wheelfeed program instructions or control signals as required. The process may be repeated until the measurements made on a test workpiece indicate that the unwanted eccentricity of each ground region is within desired limits. Thereafter the grinding machine wheelfeed may be controlled to grind further workpieces in accordance with the finally modified wheelfeed program instructions or control signals.
(d) Multiple Workpiece Measuring For Setting Up
Instead of producing only one workpiece on which measurements are made during setting up, it may be preferable at each stage to grind a number of workpieces which together form a sample, and to perform eccentricity measurements on the workpieces in each sample, and thereby determine the extent of any unwanted eccentricity both in terms of angular position and radial extent for each region of each workpiece in each sample, and to determine the mean unwanted eccentricity and mean angular position of such eccentricity for each sample, for use as the basis for the correction to the wheel feed control signals for subsequent grinding.
Where plural samples of workpieces are used to determine the corrections to be made, the process for determining what is to be the final set of control signals for the wheelfeel, can be terminated either if all of the measurements of unwanted eccentricity in all of the workpieces in a sample fall within desired limits, or if the mean of all the measurements of unwanted eccentricity fall within desired limits. The particular criterion used may depend upon the specification for the component concerned.
(e) Operational Modes of Machine Control System
According to a preferred feature of the invention, the control system for controlling the wheel feed may operate in different modes.
A first mode allows rapid advances and retraction of the grinding wheel, and rapid relative lateral movement of the wheel and the workpiece, to facilitate workpiece mounting and demounting and relative axial positioning of the wheel and the workpiece.
A second mode is provided for when the grinding wheel is grinding a concentric cylindrical region of a workpiece. Here small unwanted eccentricity can be introduced by grinding process forces, the unwanted eccentricity can be compensated by introducing small complementary forward and backward movement of the grinding wheel in step with the rotation of the workpiece so as actually to grind the cylindrical region eccentrically relative to the primary workpiece axis, but by an amount just sufficient to cancel out the process induced eccentricity.
In a third mode the grinding wheel can be advanced and retracted through considerable distances in step with the rotation of the eccentric region of the workpiece as the latter is rotated, so as to follow the much greater eccentricity of an eccentric cylindrical workpiece surface (such as the crankpin of a crankshaft).
If correction of process induced eccentricity in the eccentric cylindrical surfaces is also required, a further mode is added to the known mode, in which the wheel feed control causes the wheel to follow the excessive eccentric rotation of an eccentric workpiece region, in which the eccentric following movement is corrected by smaller backward and forward shifts of the wheel correctly in step with the workpiece rotation so as to compensate for any unwanted process induced eccentricity introduced into the eccentric cylindrical surface by the grinding process.
(f) Grinding Methods Employing the Invention
The invention also lies in a method of grinding concentric and eccentric regions of similar composite workpieces by a computer controlled grinding machine comprising the steps of setting up modified wheelfeed program instructions or control signals by any of the aforesaid methods, and thereafter when grinding cylindrical surfaces of composite workpieces similar to that used in the setting up process, using the modified wheel feed program instructions or control signals derived from the setting up procedure, to control the wheelfeed drive.
By using a high speed CBN grinding wheel, it is possible to eliminate the rough machining step typically required before a fine grinding step to produce a bearing surface, thereby allowing a two stage grinding and polishing process to produce a finished product, instead of a three stage process.
The invention thus allows the known software controlled crankpin following facility incorporated computer controlled grinding machines to permit a grinding wheel to follow the relative eccessive eccentric movement of a crankpin during grinding as the crankshaft rotates, to be utilised, albeit acting at a smaller scale, to compensate for unwanted eccentricity introduced into cylindrically ground surfaces of a workpiece which should be concentric as a consequence of grinding process forces exerted, as such surfaces are ground and which tend to appear when the workpiece is stress relieved.
This aspect of the invention thus in effect provides for grinding of eccentric cylindrical surfaces of a workpiece in known manner, and eccentrically grinding cylindrical surfaces of the same workpieces which are to be concentric about the primary workpiece axis the degree of eccentricity being such that when a ground workpiece is stress relieved after grinding, the region in fact will be concentric relative to the primary axis of the workpiece.
Whilst the invention is of particular application to the grinding of crankpins and journal bearing regions on elongate crankshafts, it is to be understood that it is equally applicable to the grinding of any composite workpiece so as to remove eccentricity which the grinding process can introduce into the workpiece, and which becomes evident as the latter becomes unstressed.
(g) Grinding Machine
A computer controlled grinding machine for performing methods of grinding as aforesaid comprises a grinding wheel, drive means for rotating the wheel, wheelfeed drive means for advancing and withdrawing the grinding wheel towards and away from a workpiece region in a precise manner, computer means for controlling the wheelfeed drive means, which computer means includes memory means for storing programmable wheel feed instructions whereby the position of the wheel at each instant during a grinding process can be controlled, and program means loaded into the computer to control the wheel feed drive and the position of the grinding wheel so as to control, at selected region along the workpiece, the advance and withdrawal of the grinding wheel before during and after grinding each said region, so that eccentric and concentric cylindrical grinding of different workpiece regions may be performed without demounting the workpiece.
Where the grinding machine introduces unwanted eccentricity into concentrically ground cylindrical workpiece surfaces, the computer means may include memory means for storing correcting or corrected wheelfeed program instructions or control signals for use during the grinding of such regions, which correcting or corrected instructions or signals are obtained from any one of the setting up processes described above.